Blood pressure (BP) refers to the force exerted by circulating blood on the walls of blood vessels, and constitutes one of the principal vital signs. The pressure of the circulating blood decreases as blood moves through arteries, arterioles, capillaries, and veins. The term blood pressure generally refers to arterial pressure, i.e., the pressure in the larger arteries. Arteries are the blood vessels which take blood away from the heart.
Blood pressure in the arteries changes in a generally oscillatory manner and can be displayed as a waveform (a graph of pressure against time). The peak pressure in the arteries is known as the systolic blood pressure (SBP) and occurs near the beginning of the cardiac cycle. The lowest pressure in the arteries, which occurs at the resting phase of the cardiac cycle, is known as the diastolic blood pressure (DBP). The average pressure throughout the cardiac cycle is known as the mean arterial pressure (MAP), and the pulse pressure (PP) is the difference between the systolic and diastolic pressures.
Existing blood pressure monitors provide a measure of systolic and diastolic blood pressure in the peripheral arteries, e.g. the arm. However, it has long been recognised that systolic blood pressure measured at the brachial artery, radial artery or digital artery exceeds central systolic blood pressure (cSBP) at the aortic root because systolic blood pressure is amplified above that close to the heart by propagation along the peripheral arteries in the upper limb as a result of reflected pressure waves. This also results in a difference in the central pulse pressure compared to the peripheral pulse pressure. Diastolic blood pressure is similar at central and peripheral sites because of the slow rate of change of pressure during diastole.
Mean arterial pressure is also similar at central and peripheral sites. cSBP would be expected to provide a better indication of the load on the heart and hence be more closely related to heart disease than peripheral systolic blood pressure (pSBP).
Blood pressure is usually measured in the upper arm by an oscillometric method using a cuff inflated around the upper arm. Pressure in the cuff is inflated to a pressure above systolic blood pressure in the arm (peripheral systolic blood pressure, pSBP) and then slowly deflated to a pressure below diastolic blood pressure in the arm (DBP). At any time during the deflation, when mean cuff pressure (MCP) during one cardiac cycle falls below pSBP, cuff pressure oscillates by a small amount around MCP. pSBP and DBP can be estimated from the amplitude of pressure oscillations within the cuff.
It is known that, whilst the mean arterial blood pressure (MAP) and DBP differ little between the aorta and conduit arteries in the arm, pSBP measured in the upper limb is amplified above central systolic blood pressure in the aorta (cSBP) by propagation along the upper limb which results in amplification due to reflections. cSBP is thought to provide a better estimate of the risk of a cardiovascular event than pSBP. Furthermore, when comparing antihypertensive drug regimes that have similar effects on pSBP but differential effects on cSBP, lower cSBP was associated with improved outcome. There is therefore a demand for methods of non-invasive estimation of cSBP.
The most commonly employed method involves the measurement of a peripheral blood pressure waveform from the radial artery by applanation tonometry (holding a pressure sensor over the radial artery to gently compress it against underlying bone). This pressure waveform may then be calibrated from oscillometric measurements of pSBP and DBP. A “generalised transfer function” (GTF) is then applied to this peripheral cuff pressure waveform to transform it into a central waveform from which cSBP can be estimated. The GTF can be derived either in the frequency domain using fast Fourier transforms or in the time domain using a parametric function. The GTF exploits the fact that, for a given shape or frequency content of the central waveform, the upper arm exerts a relatively constant influence on the waveform irrespective of age and other intra-individual characteristics. Applanation tonometry requires is performed by a trained observer, takes several minutes to perform, requires relatively expensive equipment, and requires an oscillometric or other measurement of blood pressure.
An advance on the tonometry based methodology derives a blood pressure waveform direct from an upper arm cuff used for oscillometric measurement of blood pressure. When the cuff is inflated (usually to a pressure between DBP and pSBP), pressure waveforms recorded from the cuff bear some resemblance to those within the artery within the arm (and those obtained by tonometry). The cuff can be inflated to a suprasystolic pressure (above pSBP) but this is uncomfortable for the patient and also means that the method is more difficult to apply during the routine oscillometric measurement of blood pressure. During such a measurement an objective is to minimise the time at which cuff pressure exceeds pSBP in order to maximise the information on cuff pressure oscillations during deflation from pSBP to DBP.
The GTF used for transforming a tonometer derived peripheral waveform to a central waveform cannot be used to transform the cuff waveform to a central waveform. An alternative GTF may, however, be used to transform the cuff waveform to a central waveform. The characteristics of the GTF are dependent on the mean pressure within the cuff during the acquisition of the cuff waveform. Because the cuff waveform differs from an intra-arterial or tonometer derived waveform, it cannot be calibrated from pSBP and DBP.
The central waveform derived from applying a GTF to the cuff waveform can be calibrated from the MAP and DBP because of the equality of MAP and DBP at central and peripheral sites. MAP and DBP values may be obtained by an oscillometric method.
The above methodology, therefore, allows cSBP to be determined from a blood pressure cuff during (or immediately before or after) the conventional measurement of oscillometric blood pressure and in a manner that imposes no more onerous requirements on the patient or observer than the conventional measurement of blood pressure. A first disadvantage with this blood pressure cuff approach is the requirement to calibrate the waveform from MAP and DBP. There is at present no agreed standard for validating the accuracy of MAP as derived by an oscillometric method. A second disadvantage is that calibration from MAP and DBP may be influenced to a greater degree by errors in estimation of DBP by the oscillometric method. Conversely, if a calibration by pSBP and DBP is performed, errors in estimation of pSBP give rise to a similar error in cSBP. Thus, even if the oscillometric method is subject to some error, the difference between the estimated cSBP and pSBP remains an accurate measure of the difference between the true cSBP and pSBP.
Calibration of a cuff derived waveform by pSBP and DBP, prior to transformation to a central waveform, is relatively inaccurate compared to when a similar procedure is applied to a tonometer derived waveform. This is because, unlike the tonometer derived waveform, the shape and form of the cuff waveform do not bear a constant relationship to that of the true intra-arterial pressure waveform. The cuff waveform is distorted by an amount dependent on the difference between the MCP and the intra-arterial pressure and on the phase of the cardiac cycle. A closer approximation to intra-arterial pressure is obtained by inflating the cuff to a pressure above pSBP, but this has the disadvantages discussed above.
One method of transforming a peripheral pulse waveform measured using an oscillometric method, i.e. gained using a cuff pressure device, into a corresponding intra-arterial waveform uses a general transfer function. This method is not fully accurate, thus there is motivation to improve the accuracy of the method of transforming a peripheral pulse waveform into an intra-arterial waveform.